This disclosure is directed to a circuit for operation of a photomultiplier tube (PMT) installed in logging tools lowered into oil and gas wells. It is intended for use in a logging tool which can be used both during drilling and after completion of an oil well. It can also be used to service an already existing well. The particular logging tool is the type of tool which incorporates one or more PMTs to measure scintillation events including those which may occur naturally or those which are induced. It is particularly useful for induced scintillation events where the surrounding formations are irradiated with radiation or particles and which respond with an output forming a spectral peak. It will be described in the context of forming a response to the 660 Kev spectral peak associated with cesium-137 isotope. Such an irradiation source is often used in logging tools, as for example, in tools for measuring formation density.
In a typical logging tool a source of radiation is placed in a sonde. It will typically emit neutrons or gamma radiation. In its use, long and short detectors are installed on the sonde, and as is understood by those familiar with logging, the short detector is located close to the source, and the long detector is located remote from the source. Typically, the long and short detectors are practically identical and form output signals which differ primarily in scale resulting from the difference in spacing. Accordingly, the present disclosure is directed to an improved system which can be used in both the long and short detectors.
The downhole environment encountered by a logging tool includes exposure to constantly increasing temperatures with depth. Electronic equipment is ordinarily sensitive to increases in temperature. Accordingly, a logging tool which is lowered into a deep well will change in operation, drifting from surface calibration points, and providing different outputs. One approach to overcoming this is to place certain key elements within a thermal insulating flask. Such a device, however, is costly to build, has limited interior room and heat will eventually accumulate within the flask. Another approach is to utilize expensive gain stabilization circuits. It is suggested that the present procedure is remarkedly more desirable than typical gain stabilization circuits, and it also avoids the requirement of installation in a flask to exclude external heat.
The detection and quantification of scintillation events begins with a scintillation detector, and the preferred form of detector is a solid crystal body, the preferred form being NaI. It provides a desirable response, but is sensitive to temperature. Moreover, it does not warm up evenly so that different portions of a typical measuring device featuring an NaI crystal may have temperature gradients across the crystal body; the crystal itself may impose drift on the system as temperatures change. Temperature drift and loss of stability can obscure data obtained from logging. It is particularly a problem in that the PMT operates with a high voltage supply system and involves various stages of amplification. Moreover, signal discrimination must be implemented and that level may vary over time. Not only is there drift as a result of temperature, but manufacturing variations are also introduced. Aging variations also occur. It is possible for the count rate to be several percent unstable, and this can materially impact the acquired data. This is a drift level often unacceptable because it completely obscures data quality. The present disclosure is directed to a very simple, surprisingly proficient system whereby PMT amplification in spite of temperature and aging is controlled. It involves the use of a gain stabilization circuit which is connected to the high voltage power supply for the PMT. This includes a output amplifier which applies the PMT output signal to a amplifier system connected to a set of comparators. Each comparator has an input voltage which defines a threshold value, and thereby serves as a discriminator. The output of the comparators is connected to a simple logic circuit which provides a control signal for a pulse generator connected to a digital-to-analog converter. That in turn provides an analog control signal for a high voltage power supply (HVPS) connected to the PMT. The voltage for the power supply is adjustable, and under this control, amplification is changed by changing the high voltage applied to the PMT across the cathode and anode thereof. The HVPS can be either negative or positive.